If you ever forget your root password, you can actually reset it with the following steps.

• Interrupt the boot at the GRUB stage and boot to runlevel 1, AKA single user mode.
• Interrupt GRUB by typing a character such as “space” then append to the kernel line by typing “a”
• Backspace through “rhgb quiet” and append ” 1 “
• Then hit <enter>

This will give you a root shell and not a login prompt. From there you can use the “passwd” command to set a new root password.
Other user passwords can be reset, and other administrative tasks can be performed as well. Exiting the single user root shell will boot to the multi-user mode defined in /etc/inittab.

CentOS-6 Note: Due to an upstream SELinux bug the root password cannot be reset on a fresh install of CentOS-6 without an additional step. Booting with “selinux=0” appended to the grub kernel line, or doing “setenforce 0” after logging in and before attempting to reset the password are work-arounds.

(If SELinux is enforcing then do not forget to relabel your filesystem afterwards or you will not be able to login until you reboot and repeat the instructions and include the autorelabel)

 From: http://wiki.centos.org/TipsAndTricks/ResetRootPassword